1. Field of the Invention
This invention pertains to measurement of acoustic wave travel time and more particularly to the discrimination of shear waves in a return acoustical logging signal and the determination of shear wave travel times without using an erroneously selected return cycle for such measurement.
2. Description of the Prior Art
It is well known that an acoustic wave induced into a geological formation produces a plurality of wave propagation modes which can be received and detected to give information about the formation. Two of the most useful wave mode components of such an acoustic wave are the compression wave and the shear wave. The compression wave is the strongest and most rapidly travelling of the wavefronts and results from a compression-type impact on a geological interface. The shear wave, on the other hand, is a slower moving wavefront and is a result of lateral propagation along an interface.
Although the compression wave characteristics provide much valuable information by itself, it does not provide some of the information that is revealed by the shear wave returns or by a comparison of the compression wave returns with the shear wave returns. For example, fissures that are approximately normal to an acoustic wave would not cause an appreciable compression wave attenuation but would cause appreciable shear wave attenuation. Also, fluids have a different effect on the appearance of a compression wave as compared with the appearance of a shear wave, both with respect to amplitude and travel time.
One of the important values that is provided is the velocity or travel time values of the various components of the overall acoustic wave. In the above discussion for example, it is true that different valuable information is revealed about the formation from the velocity or travel time of the compression wavefront and from the information provided by the velocity or travel time of the shear wavefront. The compression wavefront is fairly easy to recognize and to measure the speed of by the use of spaced-apart receivers since, as mentioned above, the compression wave returns are the first returns received following an acoustic impulse event induced into the formation.
It is not always so easy to recognize the shear wavefront arrival at a receiver and to take its measurement. If a wrong cycle of a return is selected as detected by the second of spaced-apart receivers compared with the cycle detected by the first receiver, erroneous and obvious misleading results are indicated.
Pickett, et al., U.S. Pat. No. 3,276,533, is directed to a method of identifying the arrival of shear wave components in two received acoustic signals in well logging operations. The method there disclosed is based on the recognition of wave components having different velocities in each of the received signals. The arrival times of the beginning of the compressional waves in the first and second received signals are detected (T.sub.11 and T.sub.12 in FIG. 4). The time delay between the time of the arrival of the compressional wave and each successive peak in the signal is computed for each signal. The ratio of the time delays for corresponding peaks (i.e., first, second, etc.) in the two received signals is observed after each successive peak detection, and the first peaks for which the ratio is significantly different from unity are labelled as the first shear wave peaks for their respective signals. A cycle skipping, of course, would cause this same result, and go undetected, in the Pickett, et al. method. Further, there is no recognition of shear waves with respect to a standard, such as with respect to the arrival of the compressional waves. As will be explained hereinafter, there is a relationship which the Pickett, et al. method does not utilize at all.
Engle, U.S. Pat. No. 3,467,875, discloses a method and apparatus for eliminating cycle skipping in acoustic well logging. A value representative of the maximum acceptable time change which can occur between successive sample time values in successive received acoustic waves is stored in a maximum delta circuit 24. The time difference between transmission and receipt of an incoming signal is compared with that of a previously validated signal stored in a digital-to-analog converter 20 to determine if the incoming signal falls within the acceptable range. If it does, it is established as the new valid signal and is recorded for logging. The Engle Patent fails to disclose the use of a predetermined relationship betweeen compressional and shear components of acoustic waves as a basis of validation of received signals.
Trouiller, et al, U.S. Pat. No. 3,900,824, discloses a method for the elimination of cycle skipping which is similar to that of the Engle patent method. In the Trouiller patent method, the maximum acceptable difference between time values of successive measurement signals is computed as a given fraction of the average period of the acoustic waves transmitted by the transmitter in the logging tool. The Trouiller patent method does not employ utilization of a predetermined relationship between compressional and shear components of acoustic waves for shear wave identification.
Elliott, et al, U.S. Pat. No. 3,390,377, utilizes at least a pair of receivers spaced apart in a borehole for receiving formation compressional and shear wave returns, adjusting the amplitude and time of the second receiver to correspond with that of the first receiver and cancelling the first returns by the second returns, the remaining returns presumably being those other than compressional waves. Although such technique may enhance the presence of shear waves, it is not the technique employed herein by Applicant and does not assure against false data being interpreted as true data because of cycle skipping.
Waters, et al, U.S. Pat. No. 3,302,164, shows the development of compressional wave induced returns using a particular type of transmitter as well as the development of shear wave induced returns using a different acoustical generator for comparison purposes. The technique may give some information about shear waves, but it does not employ the technique utilized as set forth herein.
The technique described herein employs a relationship that is known to exist in most geological formations between the travel time of compressional waves and the travel time of shear waves produced for a common impulse source of less than 15 kc. A simple sine wave impulse can be employed as the acoustical transmitted signal, but different types of such signals, and even complex signals, can be employed with the method herein described, with equal validity of result. The travel time of a compressional wave is readily determinable by observing the onset of the wave at two spaced apart receivers and by dividing the time difference results by the distance there between in terms of appropriate linear units of measurement, such as feet.
Because it is known that the range of shear wave to compression wave travel time ratios that exists for almost all geological formations, the approximate arrival of shear waves can be determined. In fact, after the compression wave velocity or travel time is known and by picking the largest number of the relationship range, it is possible to determine for a given return received sequence of cycles that no more compression wave cycles are detected after a predetermined amount of time after the initial onset. Therefore, the cycles that then occur are assumed to be shear wave cycles. By subtracting the time of arrival of such such detected shear wave cycle detected by a first receiver with the time of arrival of a corresponding detected shear wave cycle detected by a second receiver, and corresponding for the respective distances the receivers are from the transmitter that produces the impulse event causing the wave onsets, the travel time of the shear wave is determined.
This measurement is assured to be the shear wave travel time provided that its value fits within the window or limits of 1.55 to 1.9 times the compression wave travel time. If it does not fit the window, a cycle has been skipped somewhere, probably by the second receiver.
Therefore, it is a feature of the present invention to provide an improved shear wave velocity or travel time measurement of an acoustic signal by validating it with respect to the readily determinable velocity or travel time of the compression wave component thereof.
It is another feature of the present invention to provide an improved measurement of shear wave velocity or travel time in an acoustic signal that ensures against false data being employed because of signal cycle skipping.